Today, three techniques are chiefly employed so as to achieve the combination of the 2 sought-after effects: optical transparency and microwave reflectivity/absorption.
A first technique consists in associating with the optical porthole a metallic grid of the same dimension.
From a microwave frequency point of view, this grid behaves as a high-pass frequency filter whose performance is related to the size of the wires of the grid, to its pitch and to the nature of the metal used (generally gold or copper). Indeed, for wavelengths which are large in relation to the pitch of the grid and for thicknesses of penetration of the electromagnetic field (so-called skin thicknesses) that are smaller than the grid wire thickness, the grid is seen as a uniform and therefore reflecting metallic layer.
From an optical point of view, the wires of the grid behave as diffracting elements. For a given wavelength, it is the effectiveness of diffraction determined by the periodicity of the grid as well as by the size of the wires relative to the wavelength considered, which will determine the optical losses. For example, a copper grid with a wire diameter of 30 μm and with a regular mesh cell (a pitch) of 224 μm exhibits an optical transparency in the visible of 78%.
The performance of such devices is limited chiefly by technological limitations. Indeed, when the grid must be mechanically self-supported, its wire diameter and its pitch may not be reduced beyond the limits of resistance of the materials; the optical diffraction then becomes too significant.
Let us also point out the existence of bi-periodic grids implementing semi-conducting materials and making it possible to couple the incident RF wave impinging on an absorbent layer.
Another technique employed consists in depositing on the optical porthole, a transparent conducting layer. To this end, metals deposited in a fine layer or doped oxides are commonly employed (or in a thin layer, ITO, ZnO). For microwave frequencies, the semi-transparent layer behaves like a metal making it possible to uniformly reflect the waves whose frequency is below the plasma frequency. This technique is typically employed on airplane or helicopter canopies.
Though this technique may be satisfactory in the visible (notably for all applications related to viewing), such is not the case for wavelengths above 1.5 μm where, when the layer is sufficiently conducting with a surface resistance of typically less than 10Ω (sheet resistance), the IR waves are strongly reflected and/or absorbed. The typical conductivities are of the order of about ten ohms (sheet resistance) for a transmission of 40% at 1.5 μm.
Finally a third technique, different from the other two, implements a frequency-selective surface of the Salisbury screen type, with optically compatible materials.
This type of surface consists of a partially reflecting microwave frequency diopter, of a substrate of an effective thickness of λ/4, followed by a highly reflecting RF surface. Thus, the RF reflections on the first diopter are coupled in phase opposition with the reflection on the highly reflecting diopter thus giving rise to a decrease in the RF reflectivity of the porthole.
These reflecting surfaces can be produced with the aid of grids or of semi-transparent layers as presented previously.
This type of filter is effective only for a narrow frequency band. Techniques exist for widening the operating band of this type of device (Jaumann screen for example); however, they lead to the production of relatively thick structures.
Consequently, there remains to date a need for a porthole that is simultaneously satisfactory for all the aforementioned requirements, in terms of good IR transmission, low RF transmission and good mechanical resistance.